The answer to that is...Yes, mostly.
The mainstage shows range from solo guitar to full band. The instrumentation on those shows also can vary pretty widely. So a full band might be your basic 'two guitars, bass & drums' generic band...oooooooooooor it might be 'mandolin, accordion, bass balalaika, penny whistles, harmonica & trombone', and anything in between. Maybe not. But I wouldn't be surprised..
Groovelilly was a trio with a six-string electric violin, keys and hand percussion, I think. Or maybe violin, grand piano, drum kit. It's hard to remember. Another act I saw was piano and grand harp. Another had three ukeleles a guitar and a cello. Tom Prasada-Rao had three guitars, a dotar, an electric sitar, bass guitar, keys & drums for one song
But that's all on the main stage. Out in the campgrounds, it can get REALLY weird.
But mostly it doesn't. The majority of songcircles have no bass.
If they have one, it can be nearly anything. I've seen normal acoustic URB's, aluminum body URB's, washtub basses, ABG's, solidbody and semi-hollow bass guitars with battery powered amps of assorted levels of effectiveness, a bass banjo, bass balalaikas, and a couple of very cool variations on washtub basses called 'cabletubs'.
The 'cabletub' has a washtub for the body of the bass. But in stead of the mostly unattached stick that a typical washtub bass would have, it has a full URB scale neck standing up in the center of the diaphragm of the tub bottom, braced so that the neck is held in place by cables and braces like the mast of a sailboat. The strings are mounted in the center of the tub bottom and just happen to run right up the fingerboard, exactly like they would on a regular URB. It sounded amazingly like a real regular URB. The night I found out these things existed I saw two. One had three strings (EAD), and the other only had two (EA). The guy carrying the three string one had it and a guitar. I asked about it. He let me play a song on it with him on guitar. Then he promptly drafted me to be his bass player for the night. He said "You're a ral bass player. I'm a guitar player who has basses. If you stay with me tonight, it ought to be good." We played several circles. It was good.